This application is based on Japanese Patent Application No. 2001-314371 filed in Japan on Oct. 11, 2001, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a projection type display apparatus. For example, the present invention relates to the projection type display apparatus such as a liquid crystal projector for projecting and displaying a two-dimensional image of an illuminated liquid crystal panel onto a screen.
2. Description of the Related Art
A projection type display apparatus, which uses a plurality of polarizing beam splitters (PBS) for projection of an illumination to a light valve (for example, a liquid crystal panel) or a light that an image is modulated by the light valve, is conventionally known (for example, Japanese Unexamined Laid-Open Patent Publication Nos. 2000-330196, 2000-321662, etc.). Its schematic optical structure is shown in FIG. 6. In FIG. 6, the reference numeral (1) is a light source, (2a) is a first lens array, (2b) is a second lens array, (3) is a superimposing lens, (4a) is a first dichroic mirror, (4b) is a second dichroic mirror, (6R), (6G) and (6B) are field lenses, (7a) and (7b) are pre-polarizing beam splitters (pre-PBS), (8R), (8G) and (8B) are main polarizing beam splitters (main PBS), (9R), (9G) and (9B) are reflection type light valves (for example, reflection type liquid crystal panels), (11) is a cross dichroic prism, (12) is a projection lens, (13R), (13G) and (13B) are spacer glasses and AX is an optical axis.
A spatial energy distribution of a light emitted from the light source (1) is averaged by the first lens array (2a) and the second lens array (2b). Lights emitted from respective cells of the second lens array (2b) are superimposed on the light valves (9R), (9G) and (9B) by the superimposing lens (3). Meanwhile, a light emitted from the superimposing lens (3) is color-separated into lights of primary colors: red (R); green (G); and blue (B) corresponding to three primary colors by first and second dichroic mirrors (4a) and (4b).
The light of the primary color B is reflected by the first dichroic mirror (4a) and by the pre-PBS (7a), and passes through the field lens (6B). Meanwhile, the lights of the primary colors G and R are reflected by the first dichroic mirror (4a) and by the pre-PBS (7b), and are color-separated into G and R by the second dichroic mirror (4b). The light of the primary color G is reflected by the second dichroic mirror (4b), and passes through the field lens (6G). The light of the primary color R transmits through the second dichroic mirror (4b), and passes through the field lens (6R). Since polarizing directions of the lights entering the main PBSs (8R), (8G) and (8B) are previously adjusted by the reflection at the pre-PBSs (7a) and (7b), higher contrast can be obtained.
The field lenses (6R), (6G) and (6B) change the illumination light into a telecentric light flux and allow a projection light to enter a pupil of the projection lens (12) by the power of the light flux. The primary color lights of RGB which pass through the field lenses (6R), (6G) and (6B), respectively, enter the main PBSs (8R), (8G) and (8B). The main PBSs (8R), (8G) and (8B) allow polarized components (P polarized lights) which are not necessary for illuminations to the light valves (9R), (9G) and (9B) to transmit and remove polarized components, and allows only polarized components (S polarized lights) which are necessary for the illuminations to the light valves (9R), (9G) and (9B) to reflect and enter the light valves (9R), (9G) and (9B), respectively.
The respective light valves (9R), (9G) and (9B) modulate the primary color lights (S polarized lights), polarizing directions of which are aligned, by means of selective polarizing control according to display of pixels of a two-dimensional image (namely, ON/OFF for each pixel), and emit a reflected light composed of two kinds of polarized lights (P polarized light and S polarized light). The primary color lights emitted from the light valves (9R), (9G) and (9B) again enter the main PBSs (8R), (SG) and (8B), respectively. The main PBSs (8R), (8G) and (8B) reflect and remove a polarized component (S polarized light) which is unnecessary for projection and allow only a polarized component (P polarized light) which is necessary for projection to transmit and enter the spacer glasses (13R) (13G) and (13B).
The spacer glasses (13R) (13G) and (13B) are jointed to the main PBSs (8R), (8G) and (8B), respectively, and to the cross dichroic prism (11), and their gaps are kept stable and suitable so that shift of the pixels for the respective colors is prevented. The primary color lights which transmit through the spacer glasses (13R), (13G) and (13B), respectively, enter the cross dichroic prism (11) to be color-synthesized. The color-synthesized projection light in the cross dichroic prism (11) is projected onto a screen (not shown) by the projection lens (12).
The contrast which is generally obtained in the projection type display apparatus shown in FIG. 6 is determined by a relationship between an extinction ratio due the PBS system from the light source to the light valves (illumination side extinction ratio) and an extinction ratio due to the PBS system from the light valves to the screen (projection side extinction ratio). Concretely, an inferior value of both values determines the contrast of the projection type display apparatus. Therefore, in the case where the projection type display apparatus having high contrast is obtained, it is necessary that both the illumination side extinction ratio and the projection side extinction ratio show high values. Further, when the illumination side extinction ratio and the projection side extinction ratio are almost equivalent to each other, the projection type display apparatus having high contrast can be obtained most efficiently.
In addition, there are two kinds of methods, a method of taking out a necessary polarized light by means of transmission (the method using a P polarized light) and a method of taking out a necessary polarized light by means of reflection (the method using a S polarized light), in a PBS for determining the contrast of the projection type display apparatus. However, in the case of the method using the S polarized light, a high extinction ratio can not be secured with a single PBS. The reason for this will be explained below. When a transmittance of the P polarized light in PBS is Tp, a transmittance of the S polarized light is Ts, a reflectance of the S polarized light is Rs and a reflectance of the P polarized light is Rp, an extinction ratio in the case of using the P polarized light is obtained by Tp/Ts, and an extinction ratio in the case of using the S polarized light is obtained by Rs/Rp. According to PBS characteristics, since the transmittance Ts of the S polarized light can be very low, the extinction ratio Tp/Ts in the case of using the P polarized light can be high. However, since the reflectance Rp of the P polarized light cannot be as low as the transmittance Ts of the S polarized light, the extinction ratio Rs/Rp in the case of using the S polarized light is lowered.
In the projection type display apparatus shown in FIG. 6, since the illumination sides of the pre-PBSs (7a) and (7b) and the main PBSs (8R), (8G) and (8G) are used in the method of taking out a necessary polarized light by means of reflection, the high illumination side extinction ratio cannot be secured due to the above reason. Moreover, since the lights which have passed through the pre-PBSs (7a) and (7b) pass through the second dichroic mirror (4b) and the field lenses (6R), (6G) and (6B), a phase shift occurs at the time of transmission and reflection, so that a shift of the polarized light occurs. This becomes a factor which causes a drop of the illumination side extinction ratio.
Meanwhile, the projection side extinction ratio is determined by only one of the main PBSs (8R), (8G) and (8B). On the projection side, since the main PBSs (8R), (8G) and (8B) take out a necessary polarized light by means of transmission, a higher extinction ratio can be obtained in comparison with an extinction ratio on the illumination side. However, it is very difficult to obtain the high extinction ratio which is necessary to achieve the high contrast (not less than 1000:1) required for the recent projection type display apparatus by mean of one PBS. For example, unnecessary polarized light is absorbed and removed by arranging a polarizing plate (polarizing film or the like) on an optical path after the emission from the main PBSs (8R), (8G) and (8B), so that a high extinction ratio can be obtained. However, since heat, which is generated due to the absorption of the unnecessary polarized light, deteriorates a performance of the polarizing plate, high brightness cannot be obtained. Moreover, in order to improve the projection side extinction ratio, it is considered that the function for removing polarized light unnecessary for projection is given to the cross dichroic prism (11), but it is very difficult to produce a cross dichroic prism having the polarization separating function.
The present invention is devised in order to solve the above problems, and an object is to provide a projection type display apparatus having high brightness and high contrast.
In order to achieve the above object, a projection type display apparatus according to a first aspect of the present invention has light valves corresponding to plural primary colors, polarization separating means for polarizing and separating primary color lights for illuminating the light valves and primary color lights modulated by the light valves, and color synthesizing means for color-synthesizing primary color lights which are necessary for projection to a screen in the primary color lights emitted from the polarization separating means. The polarization separating means has pre-polarizing beam splitters having a function for removing polarized components which are unnecessary for illumination of the light valves, main polarizing beam splitters having a function for removing polarized components which are unnecessary for illumination of the light valves and polarized components which are unnecessary for projection to the screen in the primary color lights modulated by the light valves, and post-polarizing beam splitters having a function for removing polarized components which are unnecessary for projection to the screen in the primary color lights modulated by the light valves which are arranged in this order on an optical path for the respective primary color lights.
In addition, in the projection type display apparatus according to a second aspect of the present invention in the structure of the first invention, an optical part which causes a shift of polarized lights is not provided between the pre-polarizing beam splitters and the main polarizing beam splitters and between the main polarizing beam splitters and the post-polarizing beam splitters.
Further, in the projection type display apparatus according to a third aspect of the present invention, in the structure of the first and second aspects, the pre-polarizing beam splitters have two polarizing separation surfaces and the two surfaces are almost symmetrical with planes of incidence of the main polarizing beam splitters including an optical axis.
In the projection type display apparatus according to a fourth aspect of the present invention, in the structures of the first, second and third aspects, the pre-polarizing beam splitters and the post-polarizing beam splitters take out the polarized components necessary for the illumination and projection by means of transmission, and the pre-polarizing beam splitter and the post-polarizing beam splitter which are used for the same primary color light have the almost equivalent extinction ratio.
In the projection type display apparatus according to a fifth aspect of the present invention, in the structure of the fourth aspect, in the main polarizing beam splitters, an extinction ratio when a necessary polarized component is taken out by reflection is almost equivalent to an extinction ratio when a necessary polarized component is taken out by transmission.